The conventional railway tank car comprises an opening or “manway” on the top for loading, venting or maintenance purposes. The manway includes a sidewall or “nozzle” defining the opening, and a cover received over the nozzle and bolted on. The bolts (sometimes called “eye bolts”) are generally attached to the nozzle with respective eye-bolt lugs which attach an end of each respective bolt to the side of the nozzle in a pivoting arrangement. An end of the bolt opposite the lug is received in a slot formed in the periphery of the cover defined between a pair of “ears.” A nut and washer bear on the top surface of ears to close the cover and compress the gasket. Association of American Railroads (“AAR”) Standard M-1002, which is incorporated by reference, governs manway cover specifications. (Reference to any published standard refers to the standard in effect on the filing date of this application.)
It has been found that excessive torqueing of manway cover bolts may result in gasket deformation, sometimes referred to as “cold flow,” resulting in seal failure. Repeated excessive deformation, may reduce gasket life cycle. Over time, over-tightening may cause deformation of the manway cover itself.
Ideally, a manway cover is tightened in stages, with the bolts being tightened manually in a star pattern until a specified assembly torque is reached. Assembly torque may vary, depending on the number of bolts and the gasket material, but a typical specified assembly torque for a six bolt manway cover falls in the range of 80 ft·lb to 120 ft·lb, with a specified maximum of around 200 ft·lb. In practice, however, much greater torques are applied to the bolts, upwards of 400 ft·lb. This is because, in the field, the manway cover bolts may be machine-tightened via impact wrench in a single pass, following a circle pattern, for example. In the case of over-tightening, the pattern of stress on the gasket may be localized around the eye-bolt positions, which leads to gasket failure. In an extreme case, the ears of the manway cover become deformed.